Multilayer electroactive polymer composite material comprising carbon nanotubes

ABSTRACT

An electroactive material comprises multiple layers of electroactive composite with each layer having unique dielectric, electrical and mechanical properties that define an electromechanical operation thereof when affected by an external stimulus. For example, each layer can be (i) a 2-phase composite made from a polymer with polarizable moieties and an effective amount of carbon nanotubes incorporated in the polymer for a predetermined electomechanical operation, or (ii) a 3-phase composite having the elements of the 2-phase composite and further including a third component of micro-sized to nano-sized particles of an electroactive ceramic incorporated in the polymer matrix.

Pursuant to 35 U.S.C. §119, the benefit of priority from provisionalapplication 60/551,055, with a filing date of Mar. 9, 2004, is claimedfor this non-provisional application.

ORIGIN OF THE INVENTION

The invention was made in part by employees of the United StatesGovernment and may be manufactured and used by and for the Government ofthe United States for governmental purposes without the payment of anyroyalties thereon or therefore.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is co-pending with one related patentapplication entitled “SENSING/ACTUATING MATERIALS MADE FROM CARBONNANOTUBE POLYMER COMPOSITES AND METHODS FOR MAKING SAME” (NASA Case No.LAR-16867-1), U.S. patent application Ser. No. 11/076,460, filed Mar. 3,2005, by the same inventors as this patent application.

FIELD OF THE INVENTION

This invention relates to electroactive materials. More specifically,the invention relates to a multilayer electroactive composite materialmade from layers of electroactive materials wherein each layer istailored in terms of its dielectric, electric and mechanical properties.

SUMMARY OF THE INVENTION

An electroactive material comprises multiple layers of electroactivematerial with each layer having unique dielectric, electrical andmechanical properties that define an electromechanical operation thereofwhen affected by an external stimulus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a multilayer electroactive composite materialaccording to an embodiment of the present invention;

FIG. 2 is a side view of a multilayer electroactive composite materialaccording to another embodiment of the present invention;

FIG. 3 is a side view of a multilayer electroactive composite materialaccording to still another embodiment of the present invention;

FIG. 4 is a graph of dielectric constant as a function of carbonnanotube content for an embodiment of a 2-phase carbonnanotube/polyimide composite that can be used as an embodiment of thepresent invention;

FIG. 5 is a graph of resistance as a function of load for a 2-phasecarbon nanotube/polyimide composite that can be used as an embodiment ofthe present invention; and

FIG. 6 is a graph of strain as a function of applied electric field fora 2-phase carbon nanotube/polyimide composite that can be used as anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present exemplary embodiments provide a new class of electroactivepolymer composite materials made from layers of electroactive materials.Each layer is tailored in terms of its dielectric, electric andmechanical properties that ultimately define the layer'selectromechanical operation. By making each layer unique, the ultimateelectroactive material is functionally gradient in its electromechanicaloperation. For example, each layer could be designed to have itselectromechanical operation triggered only in response to a certainlevel (or range of levels) of external stimulus. Another option is foreach layer to have a specific performance capability (e.g., an amount ofdisplacement or voltage generated in response to an external stimulus)triggered by any level of external stimulus. Still another option is acombination of the previous two examples whereby each layer is designedfor a specific performance that is triggered only when a specific level(or range of levels) of external stimulus affect the device.Accordingly, the term “functionally gradient electromechanicaloperation,” as used herein, can refer to the material's electroactivesensitivity and/or the material's electroactive performance.

Referring now to the drawings, FIGS. 1-3 are general depictions of threedifferent exemplary embodiments for multilayer electroactive compositematerials in accordance with the present invention. In each embodiment,each of the layers is designed to provide a predetermined gradient levelof electromechanical operation. The gradient levels can be discrete orcould be overlapping without departing from the scope of the presentinvention.

FIG. 1 illustrates a multilayer electroactive composite material 10 madefrom individual layers 12-1, 12-2, 12-3, . . . , 12-N of electroactivematerial. All layers of material 10 are coupled or bonded togetheracross their tangent faces. FIG. 2 illustrates a multilayerelectroactive composite material 20 made from individual layers 22-1,22-2, 22-3, . . . , 22-N of electroactive material where adjacent layersare only partially bonded to one another as indicated at regions 24. Inthis drawing, the spaces shown between the unbonded portions of adjacentlayers can be indicative of either an actual gap or un-bonded contact.Each of materials 10 and 20 would typically be fabricated by firstmaking each individual layer and then coupling/bonding the individuallayers to one another as just described. However, another option isshown in FIG. 3 where a multilayer electroactive composite material 30is made from layers 32-1, 32-2, 32-3, . . . , 32-N of electroactivematerial that are seamlessly bonded to one another (as indicated by thedashed lines therebetween) during, for example, a sequential spray on,spin cast or solution cast fabrication. Before each sequentialfabrication step, the raw material for the layer would be altered asnecessary to provide for the specific electromechanical operationthereof. Seamless bonding of layers would occur in accordance with thecuring process associated with the particular electroactive materials.

The layers in each of the above-described materials could be made from avariety of electroactive materials. For example, each layer couldcomprise one of the new electroactive materials described in thecross-referenced patent application entitled “SENSING/ACTUATINGMATERIALS MADE FROM CARBON NANOTUBE POLYMER COMPOSITES AND METHODS FORMAKING SAME,” U.S. patent application Ser. No. 11/076,460, filed Mar. 3,2005, the contents of which are hereby incorporated in their entirety byreference. The new materials are carbon nanotube polymer composites thatprovide improved electromechanical operation when affected by someexternal stimulus. For example, the electromechanical operation can be asensing operation that involves the generation of an electrical signalin response to deformation of the electroactive material caused by achange in its physical environment (e.g., changes in noise, vibration,stress, pressure, flow, temperature, etc.). The electromechanicaloperation could also be an actuating operation that involves mechanicalmovement of the electroactive material when the material has astimulating electric potential applied thereto.

More specifically, each layer in a multilayer material of the exemplaryembodiments can be an electroactive sensing/actuating (“sensuating”)material that comprises a 2-component or “2-phase” composite material.The base material is a polymer matrix wherein the polymer comprises anelectroactive type having polarizable moieties. The remaining componentor phase comprises nanotubes incorporated in the polymer matrix.Electroactive polymers with polarizable moieties include those withasymmetrically strong dipoles. Suitable polymer classes fitting thisdescription include, but are not limited to, polyimides, polyamides,silicon-based polymers, vinyl polymers, polyurethanes, polyureas,polythioureas, polyacrylates, polyesters, and biopolymers. Thepolyimides include but are not limited to 2,6-bis(3-aminophenoxy)benzonitrile ((β-CN)APB)/4,4′oxydiphthalic anhydride (ODPA)((β-CN)APB-ODPA) and other polyimides with polarizable moieties, andpolyetherimide (e.g., the commercially-available ULTEM®). The polyamidesinclude but are not limited to odd-numbered nylons. The silicon-basedpolymers include but are not limited to silicone andpolydimethylsiloxane (PDMS). The vinyl polymers include but are notlimited to PVDF, PVDF/TrFE (copolymer of vinylidene fluoride andtrifluoroethylene), poly(vinyl alcohol) (PVA), a graft elastomer such asthat claimed in U.S. Pat. No. 6,515,077, the entire contents of whichare incorporated herein by reference, and vinyl copolymers. Thepolyacrylates include but are not limited to polymethyl methacrylate(PMMA). The biopolmers include but are not limited to polypeptides andkeratin.

The presence of strong dipoles (associated with theabove-mentioned-polymers with polarizable moieties) have led researchersand industry to attempt to construct piezoelectric sensors and actuatorsfrom these materials and blends of such polymers as disclosed, forexample, in U.S. Pat. No. 6,689,288, the entire contents of which areincorporated herein by reference.

The 2-phase electroactive materials described herein use nanotubeinclusions to improve the electomechanical response of the polymerhaving polarizable moieties. In general, such nanotubes can be based ona variety of elements, including carbon or other metallic andsemi-metallic elements. However, carbon nanotubes will be describedspecifically in the example. Such carbon nanotubes can be single-wallnanotubes (referred to as “SWNT”), or they can be nanotubes made frommultiple walls, e.g., double-wall, few-wall, multi-wall, etc., all ofwhich are referred to herein as “MWNT”.

Accordingly, the 2-phase electroactive sensing/actuating (or sensuating)composite comprises a selected polymer matrix having nanotubeinclusions. In order to produce an electroactive material that acts aseither a sensor or actuator, it has been discovered that only smallamounts of carbon nanotubes need to be incorporated into the polymermatrix. The small amounts of nanotubes used in the 2-phase compositematerials are defined herein as a volume fraction of the ultimatecomposite. For example, the volume fraction of nanotubes in theexemplary 2-phase composite is expressed as “X percent of the volume ofthe composite”. The value of “X” is arrived after consideration of thetype of operation (e.g., sensing, actuating) and the amount ofelectromechanical motion of interest for a given polymer matrix andgiven external stimulus. No specific volume fraction of nanotubes (for aparticular polymer) will define a clear transition between sensing andactuating operations. Rather, a general range of volume fraction ofnanotubes will enable the composite to behave better as either a sensoror actuator. Thus, the sensing or actuating functions of the materialfor the present embodiment can be varied and controlled by the volumefraction of nanotube inclusions.

The above-described 2-phase sensing/actuating (sensuating) polymercomposite comprises pure polymers with polarizable moieties havingnanotube inclusions. However, the layers in the multilayer compositematerial of the exemplary embodiments could also be made from, forexample, a 3-phase polymer composite wherein the three componentscomprise:

(i) a polymer matrix wherein the polymer is an electroactive type havingpolarizable moieties,

(ii) micro to nano-sized particles of an electroactive ceramicincorporated in the polymer matrix, and

(iii) carbon nanotubes incorporated in the polymer matrix.

Prior efforts to improve the electromechanical operation of purepiezoelectric polymers have focused on incorporating variouspiezoelectric ceramics (e.g., lead-zirconium-titanate or “PZT”) into thepolymers to form a, composite. However, the large dielectric mismatchbetween these two significantly different types of materials (i.e.,ceramic-to-polymer dielectric ratios on the order of 50:1 or greater)makes it difficult to pole both phases of the composite. That is, theelectric field required to pole both phases is generally much largerthan the electric field required to pole the pure ceramic phase becauseof a large dielectric mismatch.

The 3-phase sensing/actuating (sensuating) composites reduce dielectricmismatch through the use of small amounts of nanotube inclusions. As inthe 2-phase case, nanotubes utilized in the 3-phase composites can bebased on a variety of elements to include carbon and other metallic orsemi-metallic elements. Carbon nanotubes will be described specificallyin the example. The carbon nanotubes used in the 3-phase composite canbe SWNT or MWNT.

The electroactive ceramics utilized in the 3-phase composite of thepresent embodiment can be any piezoelectric ceramic that can be reducedto micro-sized or nano-sized particles while providing the appropriateelectromechanical response, thermal stability and chemical stability fora predetermined application. Such ceramics include but are not limitedto lead-zirconium-titanate (PZT), lanthanum-modified lead zirconatetitanate (PLZT), niobium-modified lead zirconate titanate (PNZT), andbarium titanate. By way of illustrative example, the electroactiveceramic PZT will be-specifically discussed.

An embodiment of the 3-phase composite comprises a selected polymermatrix with both ceramic and nanotube inclusions.

To produce a 3-phase composite that acts as either a sensor or actuator,only small amounts of nanotubes need to be incorporated in thecomposite. Similar to the 2-phase composite, the amount of nanotubeinclusions is expressed as a volume fraction of nanotubes to the totalvolume of the ultimate 3-phase composite.

Methods for making the 3-phase composite include steps for the nanotubesto be either: (i) incorporated in the polymer matrix beforeincorporation of the ceramic particles, or (ii) first mixed with theceramic particles in a solution that is then incorporated in the polymermatrix.

The three-component nature of the 3-phase composite provides for thetailoring and adjusting of composition and morphology to optimizemechanical, electrical, and electromechanical properties for sensing andactuating operations. The effects of the dielectric mismatch between thepolymer and ceramic are greatly reduced by nanotube inclusions thatserve to raise the dielectric constant of the polymer matrix in the3-phase composite even when small amounts of nanotubes are used. Thus,the amount of nanotubes used is a predetermined volume fraction thatbalances the amount required to minimize the dielectric mismatch betweenthe polymer and ceramic against the amount requisite for providing apredetermined electromechanical operation during a given application.

Multilayer electroactive composite materials of the present inventioncan be made from layers of the above-described (i) 2-phase composites,(ii) 3-phase composites, or (iii) 2-phase and 3-phase composites. Thebase polymer in each layer can be the same or different. In terms of the3-phase composites, the ceramic used in each layer can be the same ordifferent. Under this approach, each layer of the multilayer materialcan be specifically tailored in terms of its dielectric, electric andmechanical properties that define the electromechanical operation of thelayer.

EXAMPLES

Examples of 2-phase and 3-phase composite materials suitable for use inmultilayer composite materials of the exemplary embodiments will now bedescribed. The selected polymer in both the 2-phase and 3-phasematerials was an aromatic piezoelectric polyimide, β-CN APB/ODPA polymermatrix. The nanotubes used were single-wall carbon nanotubes or “SWNT”as they will be referred to hereinafter. The diameter and length of theSWNTs were approximately 1.4 nm and 3 μm, respectively. The2-phaseSWNT-polyimide composites were prepared by in situ polymerization undersonication and stirring. The density of pure polyimide was about 1.3g/cm³, and the calculated density of the SWNTs have been reportedranging from 1.33-1.40 depending on chirality. The diamine anddianhydride used to synthesize the nitrile polyimide were2,6-bis(3-aminophenoxy) benzonitrile ((β-CN)APB) and 4,4′oxidiphthalicanhydride (ODPA), respectively. To prepare the SWNT-polyimide 2-phasecomposite, the SWNTs were dispersed in anhydrous dimethyl formamide(DMF) that served as a solvent for the poly(amic acid) synthesis. Theentire reaction was carried out with stirring in a nitrogen-purged flaskimmersed in a 40 kHz ultrasonic bath until the solution viscosityincreased and stabilized. Sonication was terminated after three hoursand stirring was continued for several hours to form a SWNT-poly(amicacid) solution. The SWNT-poly(amic acid) solution was cast onto a glassplate and dried in a dry air-flowing chamber. Subsequently, the driedtack-free film was thermally cured in a nitrogen oven to obtainsolvent-free freestanding SWNT-polyimide film.

A series of SWNT-polyimide nanocomposite films were prepared with SWNTconcentrations ranging from just greater than 0.00 percent (e.g., 0.01percent) to approximately 2.0 percent volume fractions. A similarprocedure was followed to make the 3-phase SWNT-PZT-polyimide composites(having similar SWNT concentrations) where, in addition to dispersingthe SWNT in DMF before the poly(amic acid) synthesis, nano-sized PZTparticles/powders were also dispersed in DMF separately and then mixedwith SWNT-DMF and the polyimide precursor.

FIG. 4 shows the dielectric constant as a function of SWNT content for a2-phase SWNT-polyimide composite. Sensing and actuating characteristicsare strongly related to the dielectric properties and a higherdielectric constant material tends to provide greater electromechanicalresponses. The dielectric constant of the pristine polyimide was about4.0. A sharp increase of the dielectric constant value was observed whena volume fraction of SWNTs between 0.02 and 0.1 percent was added tothereby change the dielectric constant from 4.1 to 31. This behavior isindicative of a percolation transition. Percolation theory predicts thatthere is a critical concentration or percolation threshold at which aconductive path is formed in the composite causing the material toconvert from a capacitor to a conductor. FIG. 4 indicates that thepercolation threshold for this material resides between 0.02 and 0.1percent volume fraction of SWNTs. The dielectric constant increasesrapidly up to a 0.5 percent SWNT volume fraction and thereafterincreases moderately with increasing SWNT volume fraction.

In FIG. 5, the resistance of a 0.2 percent SWNT-polyimide composite filmis shown as a function of load in grams force (gf). Resistance wasmonitored using a 4-probe technique while the film was elongated intensile mode under a constant rate of load. The resistance increasednearly linearly with the applied load at a rate of 114 ohm/g. Thislinear response indicates that this SWNT-polyimide composite can be usedas a sensitive strain, load or pressure sensor. The sensitivity can betailored by controlling the SWNT concentration for a specificapplication.

FIG. 6 shows the strain of a 2.0 percent SWNT-polyimide composite as afunction of the applied electric field. The strain increased with thesquare of the applied electric field thereby indicating that the strainwas primarily due to electrostriction with negligible Maxwell effectrather than a piezoelectric response. The strain reached nearly 3% at0.15 MV/m. This result is almost an order of magnitude greater strainsimultaneous with an order of magnitude lower applied field whencompared to commercial products such as PVDF and PZT, as noted inTable 1. A strip of this material demonstrated significant displacementwhen an electric field was applied. Further, the material then returnedto the initial position when the field was removed. This type ofelectroactive response indicates that this composite can be used as anactuator.

TABLE 1 Strain for Electric Field Material Strain Electric Field PVDF0.1%  ~50 MV/m PZT 0.1%   ~1 MV/m 2% SWNT-polyimide 3%   ~0.2 MV/m

The 3-phase composite will generally need the step of poling due to thepiezoceramic incorporated therein. Accordingly, Table 2 presents theresults of poling (i) pure polyimide, (ii) polyimide with just PZTceramic inclusions, and (iii) a 3-phase composite material of polyimidehaving 0.1 percent SWNT and PZT ceramic inclusions. The remanentpolarization values, which are indicative of the piezoelectric response,indicate that adding the PZT increases the P_(r) slightly. The P_(r)value increases dramatically, however, when poling a similar content ofPZT-polyimide composite that further includes SWNTs. This resultconfirms that the presence of SWNTs raises the dielectric constant ofthe composite so that it is possible to pole the PZT particles and thepolyimide simultaneously.

TABLE 2 Remanent Polarization Material E_(p) (MV/m) P_(r) (mC/m²)Polyimide 50 7 Polyimide + PZT 50 11 Polyimide + PZT + 0.1% SWNT 50 84

The mechanical properties of the 2-phase and 3-phase composites werealso measured to assess the effect of adding the SWNTs and PZTinclusions oh the modulus of the polyimide. Test results reveal thatsignificant reinforcement occurs at temperatures below and above theglass transition temperature due to the addition of SWNTs, although alarger reinforcement effect occurs at temperatures above the glasstransition temperature.

Having various layers of electroactive composites exhibit variableelectroactive properties can be valuable in a number of sensorapplications where tunable optical and dielectric properties might berequired for optimal performance, e.g., chemical sensors, opticalwaveguides, etc. By utilizing uniquely tailored electroactive materialsto form a multilayer electroactive material, the embodiments can be usedto provide functionally gradient electromechanical operation in a singledevice. Each layer is made from a new class of carbon nanotube polymercomposite sensing/actuating materials. Sensing and actuating can betailored layer-by-layer as a function of carbon nanotube loadings. The2-phase and 3-phase carbon nanotube polymer composites described hereinby way of illustration are good candidates for material layers of theexemplary embodiments as they provide greater sensing and actuatingresponses at much lower external stimuli as compared to other knownelectroactive materials.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, means-plus-function andstep-plus-function clauses are intended to cover the structures or actsdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures. Thus, although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface, in the environment of fastening woodenparts, a nail and a screw may be equivalent structures.

1. An electroactive polymer article comprising multiple layers of electroactive material with each of said layers having unique dielectric, electrical and mechanical properties that define a predetermined electromechanical operation thereof when affected by an external stimulus, wherein each of said layers is a composite of (i) a polymer with polarizable moieties, (ii) particles of an electroactive ceramic in said polymer wherein a ratio of dielectric constant of said ceramic to dielectric constant of said polymer is at least 50:1, and (iii) an amount of carbon nanotubes in said polymer effective to provide a dielectric constant for said composite that is greater than the dielectric constant of said polymer while simultaneously providing for a predetermined electromechanical operation of said composite when said composite is affected by an external stimulus.
 2. An electroactive polymer article as in claim 1 wherein, for each of said layers, said polymer is unique.
 3. An electroactive polymer article as in claim 1 wherein, for each of said layers, said polymer is the same.
 4. An electroactive polymer article as in claim 1 wherein, for each of said layers, said polymer is selected from the group consisting of polyimides, polyamides, silicon-based polymers, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters and biopolymers.
 5. An electroactive polymer article as in claim 1 wherein, for each of said layers, said carbon nanotubes are selected from the group consisting of single-wall carbon nanotubes and multi-wall carbon nanotubes.
 6. An electroactive polymer article as in claim 1 wherein, for each of said layers, said ceramic is unique.
 7. An electroactive polymer article as in claim 1 wherein, for each of said layers, said ceramic is the same.
 8. An electroactive polymer article as in claim 1 wherein, for each of said layers, said ceramic is selected from the group consisting of lead-zirconium-titanate (PZT), lanthanum-modified lead zirconate titanate (PLZT) niobium-modified lead zirconate titanate (PNZT), and barium titanate.
 9. An electroactive polymer article comprising multiple layers of electroactive material with each of said layers having unique dielectric, electrical and mechanical properties that define a predetermined electromechanical operation thereof when affected by an external stimulus, wherein said layers comprise at least one layer of a first composite and at least one layer of a second composite, wherein said first composite is (i) a first polymer with polarizable moieties, and (ii) an effective amount of carbon nanotubes in said first polymer that provides for a predetermined electromechanical operation of said first composite when said first composite is affected by an external stimulus; and said second composite is (i) a second polymer with polarizable moieties, (ii) particles of an electroactive ceramic in said second polymer wherein a ratio of dielectric constant of said ceramic to dielectric constant of said second polymer is at least 50:1, and (iii) an amount of carbon nanotubes in said second polymer effective to provide a dielectric constant for said second composite that is greater than the dielectric constant of said second polymer while simultaneously providing for a predetermined electromechanical operation of said second composite when said second composite is affected by an external stimulus.
 10. An electroactive polymer article as in claim 9 wherein said first polymer is selected from the group consisting of polyimides, polyamides, silicon-based polymers, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters and biopolymers.
 11. An electroactive polymer article as in claim 9 wherein said second polymer is selected from the group consisting of polyimides, polyamides, silicon-based polymers, vinyl polymers, polyurethanes, polyureas, polythioureas, polyacrylates, polyesters and biopolymers.
 12. An electroactive polymer article as in claim 9 wherein said carbon nanotubes in said first polymer and said second polymer are selected from the group consisting of consisting of single-wall carbon nanotubes and multi-wall carbon nanotubes.
 13. An electroactive polymer article as in claim 9 wherein said ceramic is selected from the group consisting of lead-zirconium-titanate (PZT), lanthanum-modified lead zirconate titanate (PLZT), niobium-modified lead zirconate titanate (PNZT), and barium titanate. 